High performance accelerometers are being increasingly used for high-end accelerometer applications that require inertial grade specifications. These applications include inertial navigation/guidance systems, geophysical sensing, unmanned aircraft stabilization, robotics, earthquake detection and space micro-gravity measurement applications [1][2][3].
Capacitive accelerometers are the most commonly used acceleration sensors as they are easy and inexpensive to fabricate. They offer low power consumption, low non-linearity, and high reliability and sensitivity [4][5][6].
The performance of the MEMS accelerometers is determined by the level of the noise floor present in the system. This noise floor is mainly classified into two categories of noise: Brownian Noise Equivalent Acceleration (BNEA) and Circuit Noise Equivalent Acceleration (CNEA). The Brownian Noise can be reduced by increasing the proof mass size and decreasing the damping coefficient in the accelerometer. It is governed by the following equation [3][7].
  BNEA  =                    4        ⁢                  k          B                ⁢        TD              m  
where kB is Boltzmann's constant (J/K), T is the absolute temperature, D is the damping coefficient, and m is the proof mass. D is related to the thin film of gas trapped either between the proof mass and substrate or between the moving and fixed sensing electrodes. Most out-of-plane-axis accelerometers developed so far, utilize a lumped (solid) proof mass and as such a squeeze thin film damping is present between the proof mass and substrate which greatly reduces the quality factor of the accelerometers; thus increasing the Brownian noise. Perforations might be embedded in the lumped mass to reduce damping, but that reduces the mass size; hence, Brownian noise is still largely present. Other methods that have been used to reduce BNEA include using vacuum packaging of the accelerometer, or operating the accelerometer in a closed loop mode to maintain the mass in the rest position. However, vacuum packaging is an expensive process and the closed loop mode of operation is not desirable for low cost and low power applications. The closed loop mode also adds complexity to the read-out circuit of the accelerometer. [4][2][8].
The second type of noise, i.e. CNEA, is inversely proportional to the capacitive sensitivity (Sc) of the sensing element and is governed by the following equation [3][1].
  CNEA  =            Δ      ⁢                          ⁢              C                  m          ⁢                                          ⁢          i          ⁢                                          ⁢          n                            S      c      
where ΔCmin is the resolution of the interface circuit, and Sc is the capacitive sensitivity of the accelerometer. As the capacitive sensitivity of the accelerometer increases, the CNEA decreases. However, the electrode configurations of currently available accelerometers are not area efficient in terms of generating large capacitance change, which results in small values of the capacitive sensitivity. In the current high performance accelerometers, the electrode configurations use either parallel plates or comb-drives. The former offers fairly large capacitive sensitivity, but that comes at the expense of having significant squeeze thin damping as well as limits on the travel range of the proof mass. The latter offers highly linear measurements. However, it provides relatively small values of capacitive sensitivity as the area of capacitance cannot be largely increased as the fingers have a cantilever-style structure, making them limited in length.
Such inefficient electrode configurations and lumped (solid) proof mass structures are described in U.S. Pat. No. 7,934,423 B2, U.S. Pat. No. 7,258,011 B2, U.S. Pat. No. 7,578,189 B1, US 2012/0000287 A1, U.S. Pat. No. 8,205,498 B2, U.S. Pat. No. 7,690,255 B2, U.S. Pat. No. 6,402,968 B1, and U.S. Pat. No. 7,258,012 B2.
The challenges in having a sub-micro-g MEMS capacitive accelerometers could thus be summarized as follows:                1—A solid proof mass leads to a significant squeeze thin film damping that increases the BNEA.        2—Current electrode configurations of the MEMS capacitive accelerometers provide limited capacitive sensitivity that increases the CNEA.        